Lighter-than-air aircraft include blimps, dirigibles, and balloons, all of which are varieties of aerostats. Blimps are aerodynamically shaped balloons with propulsion units and optionally with gondolas. The Goodyear blimp is a well-known example of a blimp. Dirigibles are rigid airships where the outer gas-containing envelope, or hull, has a rigid frame, and the lifting gas is contained in one or more gas bags therein. The Hindenberg is a well-known example of a dirigible. Blimps and dirigibles are referred to collectively as airships. Balloons typically do not have lateral propulsion systems, relying instead on lifting or descending into desirably-directed air currents. Hot air balloons are well known. Lighter-than-air aircraft obtain lift from buoyant forces exerted by the atmosphere on a hull substantially filled with a light-weight gas such as hydrogen or helium. Important economies are available from airships as compared to fixed-wing aircraft which must burn fuel to generate lift.
There are various current airship designs, some of which reprise older designs with updated materials and technology and others generally based on hybrid designs using buoyant airfoils. Airships are used in a variety of applications including surveillance, observation, heavy cargo, and transportation.